Electromagnetic tripping mechanisms for circuit-breakers include as known, have associated with each of the poles of the circuit-breaker, a fixed magnetic circuit excited by the current passing through the pole and a movable magnetic member held by a return spring against a stop at a certain distance from the fixed magnetic circuit, thus defining the initial working gap of the tripping mechanism. When the current passing through the pole exceeds a certain value, the force of attraction, which is exerted between the fixed magnetic circuit and the movable magnetic member, becomes greater than the force exerted by the return spring. The movable magnetic member is then violently drawn against the fixed magnetic circuit and causes tripping of the circuit-breaker by freeing a catch in the mechanism of the circuit-breaker.
The practice is known of making the limiting operating current of these tripping mechanisms vary by acting either upon the force exerted by the return spring or upon the value of the initial working gap.
Thus, French Pat. No. 1 401 630 describes an adjusting device which enables the elongation of the return spring of the movable magnetic member to be varied. Similarly French Pat. No. 2 161 615 describes an adjusting device which enables the position of the stop of the movable magnetic member to be varied.
These devices display the disadvantage of necessitating a considerable bulk if it is required to have available a wide range of adjustment of the limiting operating current. In addition, because of manufacturing tolerances, the accuracy of the adjustment carried out by the user decreases rapidly the further that one moves away from the reference position adjusted in manufacture by the manufacturer for a defined value of the limiting operating current.
An adjusting device is likewise known from the U.S. Pat. No. 2,920,161, which acts simultaneously upon the force exerted by a return spring and upon the value of the gap but in a complicated arrangement which does not allow of any saving of space to be achieved.
The object of the present invention is to provide a device which acts equally simultaneously upon a return spring and upon the gap but which is much simpler and much flatter and enables a wide range of adjustment to be obtained of the limiting operating current with small bulk whilst ensuring good accuracy over the whole range of adjustment.